Device for detecting a fuel supply system leak during an engine overrunning phase

ABSTRACT

The device for detecting leaks in a fuel supply system for an internal combustion engine includes an electronic engine control unit (23) which tests for an engine overrunning mode, closes the injection valves and shuts off the fuel pump (11) if the overrunning mode is detected. Then this control unit (23) determines a pressure gradient (gradp) of a fuel supply line pressure measured by a pressure sensor (24) or a differential pressure gradient (grad δp) of a differential pressure between fuel supply and return lines (13,17) measured by a differential pressure sensor (25) after the injection valves (15) are closed. Then it compares the pressure gradient (gradp) or differential pressure gradient (grad δp) with a predetermined threshold (SW) or respective upper and lower thresholds (SW o , SW u ) to determine if a leak has occurred.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention is based on a device for detecting a leak in a fuel supplysystem in an internal combustion engine, and, more particularly, to adevice for detecting a leak in a fuel supply system in an internalcombustion engine during an engine overrunning and for detecting whichof several injection values is leaking.

In motor vehicles with an internal combustion engine, the fuel is pumpedout of the fuel tank with the aid of an electric fuel pump and fed tothe injection valves via fuel lines. Excess fuel typically returns tothe tank via a return line. In order that an adequate quantity of fuelwill always be available, the fuel is pumped by the electric fuel pumpat an overpressure, and the electric fuel pump is regulated in asuitable way, for instance by measuring the fuel pressure and using itfor evaluation.

In some fuel supply systems, the return line is dispensed with, and thefuel supply quantity regulation is demand-based. In both fuel supplysystems it is mandatory that a leak or defect in the fuel circulation bedetected with certainty and reliably, because otherwise, on the onehand, escaping fuel or incident fuel vapors could lead to emissions thatexceed legal limits; on the other, defective injection valves mightcause engine damage, if fuel were unintentionally able to reach anengine cylinder through the leaking injection valves.

In response to this problem, methods and apparatus have already beendisclosed with whose aid leaking injection valves can be detected, orwith whose aid leak detection is carried out in conjunction with a tankventing system. Such methods and apparatus are described for instance inGerman published, unexamined patent applications DE-OS 42 43 178 orDE-OS 40 40 896. In DE-OS 42 43 178, a method is disclosed in whichleaking injection valves are detected by also firing cylinders intowhich fuel has not yet been injected, each time the engine is started.If such ignitions cause the rpm to increase perceptibly, then theassociation valve must be leaking, allowing fuel to enter the affectedcylinder.

In the method and apparatus for checking the intactness of a tankventing system as disclosed in DE-OS 40 40 896, a test is done to findwhether the negative pressure building up in the tank varies in apredictable way after the tank venting valve is opened. To that end, theentire system is monitored for functionality, and the system is assessedas nonfunctional if the negative-pressure buildup gradient is below apredeterminable threshold value.

Both known systems for detecting leaks in a portion of the fuel supplysystem of an internal combustion system are either rather complicated,or fail to guarantee that leak detection is possible throughout theentire fuel supply system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device fordetecting a leak in a fuel supply system of an internal combustionengine of the above-described kind, which does not suffer from theabove-described disadvantages.

It is also an object of the present invention to provide a device andmethod for detecting a leak in a fuel supply system of an internalcombustion engine of the above-described kind during an engineoverrunning phase of engine operation.

According to the invention the device for detecting a leak in a fuelsupply system for an internal combustion engine during special operatingconditions including an engine overrunning mode of the internalcombustion engine comprises

a pressure sensor for continuously generating a pressure sensor signalindicative of a fuel supply line pressure in a fuel supply line of adifferential pressure sensor for continuously generating a differentialpressure sensor signal indicative of a differential pressure between thefuel supply line and a fuel return line when the latter in present; and

an engine control unit connected with the pressure sensor to receive thepressure sensor signal or with the differential pressure sensor toreceive the differential pressure signal in the case in which a fuelreturn line is present. The engine control unit includes means fordetecting an engine overrunning condition, means for closing injectionvalves of the engine when the engine overrunning condition of detected,means for shutting off the fuel pump when the injection valves areclosed, means for determining a pressure sensor signal of a differentialpressure gradient of the differential pressure form the differentialpressure sensor signal after the injection valves are shut off and meansfor comparing the pressure gradient to a predetermined pressure gradientthreshold or for comparing the differential pressure gradient torespective lower and upper predetermined differential pressure gradientthresholds and means for signaling a presence of the leak when thepressure gradient is greater than the predetermined gradient thresholdstored in the engine control unit or when the differential pressuregradient is less than the lower differential pressure gradient thresholdor greater than the upper different pressure gradient threshold.

The device according to the invention has the advantage over the priorart that the entire fuel supply system can be monitored for intactness.The simplicity of the device according to the invention is especiallyadvantageous; in most conventional fuel supply systems, it can beemployed without using additional expensive components.

This advantage is attained in that the fuel pressure is measured using apressure sensor, and that after the electric fuel pump is switched off atest is done to find whether the overpressure in the fuel supply systemvaries in a predeterminable way. If not, it can be ascertainedimmediately that there is a leak or defect in the fuel supply system.

Additional advantages of the invention are attained by the provisionsrecited in the dependent claims. It is especially advantageous that theleak detection can be carried out in the form of an "on-board diagnosis"and is executed in the control unit of the motor vehicle. It is alsoadvantageous that the leak detection can be employed both in fuel supplysystems with a return line and in those without a return line.

In conjunction with other evaluations and error detections that areperformed in the control unit, it is possible to detect precisely whichinjection valve is leaking. This can be done for instance by combiningthe leak detection with a method of detecting combustion misfiring.

The leak detection is done under special engine operating conditions,such as when the control unit detects an engine overrunning phase, inwhich during the overrunning shutoff the engine is no longer suppliedwith fuel through the injection valves. During the overrunning shutoff,the electric fuel pump is then shut off, after which the leak diagnosisis then performed.

Leak diagnosis during overrunning has the advantages that it can beperformed more often and can be repeated during the same trip, and thatcontinued operation of the control unit after engine shutoff is notrequired in order to perform the diagnosis.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the invention is shown in the drawing andwill be explained in further detail in the ensuing description. The soleFIGURE of the drawing schematically shows the essential components of afuel supply system in which the leak detection according to theinvention can be performed.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The components of a fuel supply system of an internal combustion enginethat are essential in order to understand the invention are shown in thedrawing. The fuel tank is shown at 10; the fuel pump 11, typically anelectric fuel pump, and a check valve 12, which by way of example isintegrated with the fuel pump 11, are located in the fuel tank.

From the fuel pump 11, the feed line 13 leads to the fuel distributor14, by way of which the fuel is delivered to the injection valves 15.The fuel pressure is regulated with the aid of the pressure regulator16, which in a fuel supply system having a fuel return line 17communicates with the fuel tank 10 via the return line 17 and optionallyvia a pressure holding valve 18, and which optionally communicates withthe intake tube of the engine via a connection 28.

For venting, the fuel tank 10 communicates with the engine intake tube21 via a line 19 in which a venting valve 20 is located. An adsorptionfilter 22, in which the fuel vapors are trapped, is also present betweenthe fuel tank 10 and the venting valve 20.

For controlling or regulating the engine, the control unit 23 is used,to which the requisite variables are supplied and which outputs therequisite trigger signals. The necessary calculations are performed inthe control unit. The leak detection according to the invention islikewise performed in the control unit.

The variable needed for the control or regulation is measured usingsuitable sensors. The drawing shows, first, a pressure sensor 24, whichmeasures the pressure p of the fuel in the distributor 14. As analternative to the pressure sensor 24, a differential pressure sensor 25may be provided, which communicates with the return line 17 via a line26 and measures the differential pressure δp. As connections by way ofwhich the triggering operations are effected, the drawing shows both theconnection between the control unit 23 and the fuel pump 11 and theconnection between the control unit and the venting valve 20, whichvalve is closed or opened depending on the prevailing conditions. Aconnection may also be present between the control unit 23 and thepressure regulator 16.

The fuel supply system leakage diagnosis can proceed, with the deviceshown in the drawing. The leakage diagnosis can be done under thefollowing peripheral conditions:

1) As soon as the control unit detects an engine overrunning phase, inwhich during the "overrunning shutoff" of the injection valves 15 nofurther fuel is delivered to the engine. The fuel pump 11 is then turnedoff and the test is performed.

2) If the control unit has a so-called "continued operation mode"--thatis, if it remains on for a certain "holding period" after then engine isturned off--then the diagnosis can be performed during this holdingperiod, after which the fuel pump 11 is switched off.

3) If the control unit detects an engine overrunning phase, anoverrunning shutoff can be effected, during which no further fuel issupplied to the injection valves. During the overrunning shutoff, thefuel pump can then be turned off.

The check valve 12 in the fuel loop, which by way of example isintegrated with the fuel pump 11, still maintains the system pressure pfor some time after the fuel pump is turned off. In the fuel supplysystem, the pressure sensor 24, which measures the fuel pressure p, islocated on the pressure side of the check valve 12. Even after the fuelpump 11 is turned off, this pressure is measured and evaluated in thecontrol unit. From the pressure course obtained, the control unit, orthe microprocessor contained in the control unit, can ascertain thepressure gradient. This pressure gradient, which serves as a standardfor the change in pressure, can be determined by one of the knowngradient ascertaining methods; for example, the pressure gradient can beascertained from two pressure values that are ascertained at apredeterminable time interval from one another.

If the pressure gradient ascertained is steeper than a threshold SW,which should be defined in some suitable way, then the control unitconcludes that there is a defect or leak, and some indication or display27, for instance, can be made, or warning signals can be emitted.

As the pressure sensor 24, a sensor with an analog starting variable canbe used, but it is also possible to use a contact switch, in which thepressure gradient gradp over the time between when the fuel pump isswitched off and the switching instant of the switch, if the systempressure fails to attain a predeterminable threshold, is determined.

The pressure sensor 24 is preferably located at the point indicated inthe drawing, but it may be located elsewhere instead. If fuel supplysystems without a return line, the entire fuel system may be monitored,if the pressure sensor 24 is installed at the point shown in thedrawing.

Fuel supply systems with a fuel return 17 may also be monitored in theirentirety, i.e. in their return region as well, if in addition to thecheck valve 12 in the fuel pump 11, a pressure holding valve 18 ismounted at the opening where the return line enters the fuel tank. It isthen possible to monitor not only the feed region but the return regionas well, until it enters the fuel tank. As the pressure sensor, adifferential pressure sensor 25 is used in this case; it measures thedifferential pressure δp between the fuel feed line and the return line,for instance at the fuel pressure regulator. If there are leaks in thefeed line, then after the fuel pump is shut off the differentialpressure gradient grad δp (where δp =[Pfeed-Preturn]), again ascertainedin the control unit, is initially less than in intact systems, while ifthere are leaks in the return line, the differential pressure gradientgrad δp after the fuel pump is shut off is initially greater than inintact systems. By evaluating the differential pressure gradient δp andcomparing it with two upper and lower threshold values SW_(o) andSW_(u), the leak detection can be performed in the control unit, and asuitable indication or display can be provided.

In a system having an internal combustion engine in which combustionmisfiring detection is performed in the control unit anyway, it ispossible by the combination of detecting combustion misfiring at aparticular injection valve and a simultaneously detected leak in thefuel system to conclude which injection valve is leaking, and thereby toprevent possible engine damage, or to locate the defective valvecorrectly right away.

The leak detection can be performed by the control unit every time afterthe fuel pump is shut off, or it is also possible for this detection tobe done only under specifiable conditions, such as only after the fuelpump has been off for a relatively long time. Since the method for leakdetection is performed in the engine control unit, this kind ofdetection is "on-board diagnosis". The detection of an existing leak inthe fuel supply system, or directly in an injection valve, can be storedin a memory of the control unit 23 and is then available as informationthe next time the vehicle is taken in for repair.

If the leak detection is done after the fuel pump is turned off and thusalso after the engine is turned off, then continued operation of thecontrol unit for a sufficiently long time is necessary; that is, thecontrol unit must continue to be supplied with voltage after the engineis turned off, so that the requisite functions can still proceed. If thediagnosis is to be done during overrunning, then the fuel pump must becapable of being triggered by the control unit directly, that is,independently of the ignition key.

I claim:
 1. A device for detecting a leak in a fuel supply system for aninternal combustion engine during special operating conditions includingan overrunning mode of the internal combustion engine, said internalcombustion engine including injection valves (15) through which fuel issupplied thereto and said fuel supply system with a fuel tank (10), afuel pump (11) and a fuel supply line (13) with or without a fuel returnline (17) delivers the fuel via said fuel supply line at a fuel pressureto said injector valves (15), said device for detecting said leakcomprisinga pressure sensor (24) for continuously generating a pressuresensor signal indicative of said fuel pressure in said fuel supply line(13) or a differential pressure sensor (25) for continuously generatinga differential pressure sensor signal indicative of a differentialpressure between said fuel supply line (13) and said fuel return line(17) when said fuel supply system has said fuel return line; and anengine control unit (23) connected with said pressure sensor (24) toreceive said pressure sensor signal or connected with said differentpressure sensor (25) to receive said differential pressure signal whensaid fuel supply system has said fuel return line, wherein said enginecontrol unit (23) includes means for detecting an engine overrunningcondition, means for closing said injection valves (15) when said engineoverrunning condition is detected; means for shutting off said fuel pump(11) when said injection valves (15) are closed; means for determining apressure gradient (gradp) of said fuel pressure from said pressuresensor signal or a differential pressure gradient (grad δp) of saiddifferential pressure from said differential pressure sensor signalafter said injection valves (15) are shut off; and means for comparingsaid pressure gradient (gradp) to a predetermined pressure gradientthreshold (SW) or said differential pressure gradient (grad δp) torespective lower and upper predetermined differential pressure gradientthresholds (SW_(o), SW_(u)); and means for signaling a presence of aleak when said pressure gradient (gradp) is greater than saidpredetermined gradient threshold (SW) or when said differential pressuregradient (grad δp) is less than said lower differential pressuregradient threshold (SW_(u)) or greater than said upper differentialpressure gradient threshold (SW_(o)).
 2. The device as defined in claim1, wherein said engine control unit (23) includes said means fordetermining said pressure gradient (gradp) and said means for comparingsaid pressure gradient (gradp) to said predetermined pressure gradientthreshold (SW).
 3. The device as defined in claim 1, wherein said enginecontrol unit (23) includes said means for determining said differentialpressure gradient (grad δp) and said means for comparing saiddifferential pressure gradient (grad δp) to said respective lower andupper predetermined differential pressure gradient thresholds (SW_(o),SW_(u)).
 4. The device as defined in claim 3, further comprising apressure holding valve (18) arranged at an entrance of fuel return line(17) to said fuel tank (10) to enable diagnosis of said fuel returnline.
 5. The device as defined in claim 1, wherein said internalcombustion engine includes a plurality of cylinders each having at leastone of said injection valves and said engine control unit (23) includesmeans for detecting a misfiring in each of said cylinders and means forcombining misfiring detection and leak detection in order tounequivocally determine if one of said injection valves is leaking andwhich of said injector valves is leaking.
 6. A method of detecting aleak in a fuel supply line or fuel return line in a fuel supply systemof an internal combustion engine during an engine overrunning mode, saidinternal combustion engine having a plurality of cylinders each equippedwith at least one injection valve and said fuel supply system includinga fuel tank, a fuel pump and the fuel supply line with or without thefuel return line, said method including the steps of:a) measuring apressure in the fuel supply line (13) by means of a pressure sensor (24)or a differential pressure between said fuel supply line (13) and saidfuel return line (17) by means of a differential pressure sensor (25)when said fuel supply system has said fuel return line; b) detecting anengine overrunning condition in an electronic engine control unit (23)of the internal combustion engine; c) closing said injection valves (15)when said engine overrunning condition is detected; d) shutting off saidfuel pump (11) when said injection valves (15) are closed; e)determining a pressure gradient (gradp) of said fuel supply linepressure from said pressure measured by said pressure sensor (24) or adifferential pressure gradient (grad δp) of said differential pressuremeasured by said differential pressure sensor when said fuel supplysystem has said fuel return line, after said injection valves (15) areshut off; f) comparing said pressure gradient (gradp) to a predeterminedpressure gradient threshold (SW) or comparing said differential pressuregradient (grad δp) to respective lower and upper predetermineddifferential pressure gradient thresholds (SW_(o), SW_(u)); and g)signaling a presence of a leak when said pressure gradient (gradp) isgreater than said predetermined gradient threshold (SW) stored in saidengine control unit or when said differential pressure gradient (gradδp) is less than said lower differential pressure gradient threshold(SW_(u)) or greater than said upper differential pressure gradientthreshold (SW_(o)).
 7. The method as defined in claim 6, furthercomprising testing for a misfiring in each of said cylinders and afterdetecting a misfiring combining misfiring detection and leak detectionin order to unequivocally determine if one of said injection valves isleaking and which of said injector vales is leaking.